I've been trying to figure this out for a looooong time. My motor vibrates at around 1800 and around 2400-2500 rpm. This is regardless of whether moving, sitting still, clutch in or out, whatever, it always does it. The weird thing is that if you put your hand on the top of the intake and hold it at 1800 or 2500 rpm, you don't feel the motor shaking, but you feel it if you're sitting in the car.
I've got good compression across the board, just put new plugs and wires on (but this was a problem BEFORE that, so I don't have a bad plug or wire), I've run fuel injector cleaner through it, and bought some sea-foam but haven't used it yet. The car also isn't setting off any codes. It's not the headers hitting the underside of the car (already dealt with that) because it's a different type of vibration. No piston slap, it's not burning oil, no excessive valvetrain noise, maintains good oil pressure (haven't checked fuel pressure). Motor is not throwing any codes now, but it was before I put in new plugs/wires because I had a bad wire on the #2 cylinder.
The car has recently had a new T-56 tranny put in using a poly mount. Motor mounts looked OK from visual inspection, but I didn't take them off to get a real good look. I have a new fuel filter I'm about to go put in right now.
That's all the background info. I'm really confused as to what is going on here.
I know vibrations are the hardest thing to diagnose on a car, but I was wondering if anybody had any suggestions on where to look for a solution. My thoughts were a)motor mounts b)intake is dirty and needs to be cleaned w/ seafoam c)something to do with my tuning.
Sorry for the long-*** post, any help is appreciated,